Marine seismic data acquisition and processing techniques are used to generate a profile (image) of a geophysical structure (subsurface) under the seafloor. This profile does not necessarily provide an accurate location for oil and gas reservoirs, but it may suggest, to those trained in the field, the presence or absence of oil and/or gas reservoirs. Thus, providing better image of the subsurface is an ongoing process and this process is an important tool for any oil and gas company that drill wells for exploring the oil and gas.
For a seismic gathering process, as shown in FIG. 1, a marine seismic data acquisition system 100 includes a survey vessel 102 towing a plurality of streamers 104 (one shown) that may extend over kilometers behind the vessel. One or more source arrays 106 may also be towed by the survey vessel 102 or another survey vessel (not shown) for generating seismic waves 108. Conventionally, the source arrays 106 are placed in front of the streamers 104, considering a traveling direction of the survey vessel 102. The seismic waves 108 generated by the source arrays 106 propagate downward and penetrate the seafloor 110, eventually being reflected by a reflecting structure 112, 114, 116, 118 at an interface between different layers of the subsurface, back to the surface 119. The reflected seismic waves 120 propagate upward and are detected by detectors 122 provided on the streamers 104. Although FIG. 1 shows the detectors 122 outside streamer 104, most traditional streamers house the detectors inside the streamer. The detectors may be hydrophones, geophones, accelerometers, optical fibers, etc. This process is generally referred to as “shooting” a particular seafloor 110 area.
A source array (or simply source in this document) is understood in this application to mean a plurality of source elements (typically air guns, but also possible vibratory elements or other known seismic wave generators) structured in a controlled way to have a certain geometry. The source array is towed by a vessel such that this geometry is preserved. FIG. 2 shows a bird view of a source array 206 that is towed by a streamer vessel 102. A streamer vessel is a vessel that tows streamers and, optionally, sources. A source vessel is a vessel that tows only sources, no streamers.
Source array 206 includes three sub-arrays 230, 240, and 250 (the source array may have more or less sub-arrays). Sub-array 230 includes a float 232 that floats at or below the water surface, and source elements (e.g., air guns) 234 connected to the float. Each sub-array has a similar geometric configuration. Source elements 234, 244, and 254 are distributed along the floats to have a certain geometry, e.g., they are distributed on a regular grid 260 having horizontal and vertical lines substantially perpendicular to each other. This geometry is maintained when the source array is towed by the vessel 102.
In a typical marine acquisition system, streamer vessel 102 tows two source arrays 206A and 206B, and plural streamers 104 as illustrated in FIG. 3. Each source array has the structure indicated in FIG. 2. Note that the traditional configuration is to have both source arrays having the same inline coordinate X1 and all the streamers to have their heads located at the same inline coordinate X2. The inline axis X indicates the vessel's travel direction.
The traditional seismic marine acquisitions share a common trait: a constraint imposed by the relationship between the number of source arrays, vessel speed, shot sampling and record length. Usually, temporal overlap between shot records is avoided, i.e., the firing time between consecutive shots is such that the shot records (from multiple shots) do not interfere in time with each other. A shot record is a collection of seismic data recorded in a given time window as a consequence of a single shot. No other shot is fired during the given time window. As a consequence, the source sampling is limited on the survey area (because it is not possible to shot the source more often than the given time window). A process for improving the source sampling requires significant extra acquisition time. Thus, due to the high cost of marine seismic acquisition, it is common practice to acquire data with a limited density of surface location.
A known way for increasing the shot point density is to fire the source arrays simultaneously, i.e., to allow temporal overlap between different shot records. Note that the term “simultaneously” is used herein to mean that two or more source arrays are shot exactly at the same instant T or they are shot within a time interval around instant T, where the time interval is significantly shorter than a record length of seismic data (e.g., the time interval is around 5 s and the record length of seismic data is around 10s, other values are possible). In case of simultaneous source acquisition, the recorded seismic data are blended (i.e., a seismic detector detects at the same time seismic waves originating from the two or more source arrays) and dedicated deblending algorithms are applied during the processing phase in order to separate the contribution of each seismic source array.
Accordingly, it would be desirable to provide systems and methods that increase the shots density and do no increase the acquisition time of the collected seismic data.